Within, for example, the printing and packaging industries, it is common practice to control working operations on a continuous material web by providing guide markings by means of printing on the web, these guide markings being sensed by photocells or other optical devices. These markings or control markings are often printed in a colour tone which makes a clear contrast with its surroundings, preferably black, and is applied on such panels or areas of the material web where no other printed markings or patterns occur. The markings may also be provided by means of magnetically sensible material or with the aid of mechanically sensible markings such as, for example, holes, crease lines or slots.
These guide markings are employed, for example, in the printing of multi-colour print, to adapt the position of the patterns printed with different colours to one another so that the different colour patterns will lie exactly superposed on one another. Another similar field of use is when the intention is, on a previously ready printed material web, to add print at a predetermined place in the print pattern, for example a best before date or the like or when the intention is to provide a printed pattern and a crease line pattern which facilitate fold forming, in register with each other.
Further fields of use are, for example, in those cases where the intention is, in a filling machine or the like, to advance a packaging material web an exact pattern length in order, on the one hand, to have the printed pattern in the same position on all packages, and, on the other hand, to cause the above mentioned crease line pattern to register with the forming devices of the filling machine so that the folding of the material takes place along the crease lines which are predetermined in position.
One example of such a guide marking is described in EP-A-131 241. One drawback inherent in this type of guide marking is that the marking takes up a certain area, which cannot then be provided with decorative artwork. Another drawback is that mechanical action runs the risk of scratching the mark, with the result that the detector intended for the purpose does not correctly register the guide marking. A further problem which may arise in the employment of the above-mentioned type of guide marking is that the guide marking is used for controlling the creasing machine and that the guide marking is then also used for controlling the filling machine. This implies that any possible tolerance errors in the positioning of the crease line pattern in relation to the guide marking and on the detection and forming by the filling machine may, in the worst case scenario, be added to one another, which could result in the filling machine attempting to fold the packaging material at the incorrect place in relation to the crease line pattern. The above-outlined problems are minimised according to current technology by means of finely tuned manufacturing machines and filling machines and by means of caution in the handling of the packaging material.
In this context, mention should also be made of EP-A-317 879 which briefly describes the employment of a magnetic guide marking in the form of magnetic strips placed on the packaging material. The publication relates to the design of the magnetic detector and is totally silent as to the design and construction of the magnetic guide marking. The magnetic strip placed on the packaging material is, precisely like the optically detectable guide marking, associated with problems in respect of tolerance errors in positioning and in subsequent processing operations of the packaging material. In addition, it may also be subjected to scratching. Moreover, there are numerous magnetic sources of disruption in a filling machine of conventional type, which impedes a correct detection of the position of the packaging material in the filling machine. The latter problem has been addressed in the last-mentioned publication in that the detector has been given a specific design.